For a few years, he played it coy. In a 2008 interview with New York Magazine, he sloughed off a question about who he was voting for: "I'm not the guy to ask about politics. I'm a gag writer." In 2010, he told PBS's Charlie Rose he'd only offer his opinion about President Obama off-camera.

But spend five minutes with Mr. Mamet and you realize that coy can only last so long. "Being a rather pugnacious sort of fellow I thought, as Albert Finney says in 'Two for the Road': 'As I said to the duchess, 'If you want to be a duchess, be a duchess. If you want to make love, it's hats off.'"

Hats off, indeed. Now Mr. Mamet has written a book-length, raucous coming-out party: "The Secret Knowledge: On the Dismantling of American Culture." (If only the Voice editors had been around to supply a snappier title.)

Hear him take on the left's sacred cows. Diversity is a "commodity." College is nothing more than "Socialist Camp." Liberalism is like roulette addiction. Toyota's Prius, he tells me, is an "anti-chick magnet" and "ugly as a dogcatcher's butt." Hollywood liberals—his former crowd—once embraced Communism "because they hadn't invented Pilates yet." Oh, and good radio isn't NPR ("National Palestinian Radio") but Dennis Prager, Michael Medved and Hugh Hewitt.

The book is blunt, at times funny, and often over the top. When I meet the apostate in a loft in Manhattan's Greenwich Village, he's wrapping up a production meeting. "Bye, bye, Bette!" he calls to the actress walking toward the elevator. That'd be Bette Midler. Al Pacino gets a bear hug. The two are starring in an upcoming HBO film about Phil Spector's murder trial. Mr. Mamet is directing and he looks the part in a scarf, black beret and round yellow-framed glasses. Looking out the window at NYU film school, where he used to teach, I ask him to tell me his conversion story.

He starts, naturally, with the most famous political convert in modern American history: Whittaker Chambers, whose 1952 book, "Witness," documented his turn from Communism. "I read it. It was miraculous. Extraordinary hero-journey of this fellow that had to examine everything he believed in at the great, great cost—which is a cost I'm not subject to—of abandoning his life, his sustenance, his friends, his associations, and his past. And I said, 'Oh my God. . . . Perhaps it might be incumbent upon me to see if I could get my thought and my actions into line too."

There were other books. Most were given to him by his rabbi in L.A., Mordecai Finley. Mr. Mamet rattles off the works that affected him most: "White Guilt" by Shelby Steele, "Ethnic America" by Thomas Sowell, "The Instincts of the Herd in Peace and War" by Wilfred Trotter, "The Road to Serfdom" by Friedrich Hayek, "Capitalism and Freedom" by Milton Friedman, and "On Liberty" by John Stuart Mill.

Before he moved to California, Mr. Mamet had never met a self-described conservative or read one's writings. He'd never heard of Messrs. Sowell or Steele. "No one on the left has," he tells me. "I realized I lived in this bubble."

One never tires of that curious chrysalis by which folks grow into their conservatism. The reactionary phase is certainly the most entertaining, but not the most rewarding.